An ink jet printer produces images on a receiver by ejecting ink droplets onto the receiver in an imagewise fashion. The advantages of non-impact, low-noise, low energy use, and low cost operation in addition to the capability of the printer to print on plain paper are largely responsible for the wide acceptance of ink jet printers in the marketplace.
It is known that high quality printing by an ink jet printer requires repeated ejection of ink droplets from ink nozzles in the printer's printhead. However, some of these ink nozzles may malperform, and may eject droplets that do not have the desired characteristics. For example, some malperforning nozzles may eject ink droplets that have an incorrect volume, causing the dots produced on the page to be of an incorrect size. Other malperforming nozzles may eject drops with an improper velocity or trajectory, causing them to land at incorrect locations on the page. Also, some malperforming nozzles may completely fail to eject any ink droplets at all. When such malperforming nozzles are present, undesirable lines and banding artifacts will appear in the printed image, thereby degrading image quality.
Malperforming and inoperative nozzles may be caused, for example, by blockage of the ink nozzle due to coagulation of solid particles in the ink. Techniques for purging clogged ink nozzles are known. For example, U.S. Pat. No. 4,489,335 discloses a detector that detects nozzles which fail to eject ink droplets. A nozzle purging operation then occurs when the clogged ink nozzles are detected. As another example, U.S. Pat. No. 5,455,608 discloses a sequence of nozzle clearing procedures of increasing intensity until the nozzles no longer fail to eject ink droplets. Similar nozzle clearing techniques are disclosed in U.S. Pat. No. 4,165,363 and U.S. Pat. No. 5,659,342.
Another reason for nozzle malperformance may be due to failures in electric drive circuitry which provides a signal that instructs the nozzle to eject a drop of ink. Also, mechanical failures in the nozzle can cause it to malperform, such as failure of the resistive heating element in thermal inkjet printer nozzles. Nozzle clearing techniques as described above cannot repair failed resistive heaters or failed electric driver circuits which, may cause nozzles to permanently malperform. Of course, presence of such permanently malperforming or inoperative nozzles compromises image quality.
U.S. Pat. No. 5,124,720 to Schantz and European Patent Application EP 0855270A2 to Paulsen et al disclose methods of printing with an inkjet printhead even though some of the nozzles have failed permanently. As understood, these methods provide for disabling portions, or "zones", of the printhead that contain failed nozzles, and printing with the remaining zones containing functional nozzles. However, these methods are disadvantaged in that if all zones contain a failed nozzle, then correction is not possible. Also, the presence of any failed nozzles will increase the printing time considerably.
Other methods of compensating for malperforming nozzles are known that utilize multiple print passes. The concept of using multiple print passes to improve image quality is disclosed in U.S. Pat. No. 4,967,203 to Doan et al. In this method, which is referenced for its teachings, the image is printed using two interlaced print passes, where a subset of the image pixels are printed on a first pass of the printhead, and the remaining pixels are filled in on the second pass of the printhead. The subset of pixels is defined such that the pixels are spatially dispersed. This allows time for the ink to dry before the remaining pixels are filled in on the second pass, thereby improving image quality. Printing images using multiple print passes has another benefit in that for each nozzle there is at least one other nozzle that is capable of printing along the same path during the next (or previous) pass. This is used advantageously by Wen et al in the above cross referenced patent application, which discloses a method for compensating for failed or malperforming nozzles in a multipass print mode by assigning the printing function of a malperforming nozzle to a functional nozzle which prints along substantially the same path as the malperforming nozzle. This is possible when the functional nozzle is otherwise inactive over the pixels where the malperforming nozzle was supposed to print. However, this technique does not apply when it is required that ink be printed at a given pixel by more than one nozzle. In high quality inkjet systems, this is often desirable, as described hereinbelow.
To further improve image quality, modern inkjet printers provide for new ways of placing ink on the page. For example, several drops of ink may be deposited at a given pixel, as opposed to a single drop. Additionally, the plurality of ink drops placed at a given pixel may have different drop volumes and/or densities. Examples of these high quality inkjet systems are disclosed in U.S. Pat. Nos. 4,560,997 and 4,959,659. Each particular way that ink can be placed at a given pixel by one pass of a nozzle is called a "state". Different states may be created by varying the volume and/or density of the ink drop. The reason that this is done is that increasing the number of states in an inkjet printer increases the number of density levels that can be used to reproduce an image, which increases the image quality. For example, consider a binary inkjet printer that can place at each pixel either no drop or a single large (L) drop of fixed volume and density during a single print pass. This printer has only two states (per color), denoted as: {0} and {L}. Correspondingly, this binary printer has only 2 fundamental density levels, and the intermediate densities are achieved by halftoning between the two available states. Now consider a modern inkjet printer that can print either no drop, a small drop (S), or a large drop (L) of a fixed density. This modern printer has three states: {0}, {S}, and {L}. Taking this one step further; if the modern inkjet printer prints in a 2 pass interlaced mode, as discussed earlier, then two states can be placed at any given pixel. The number of fundamental density levels will be equal to the number of combinations of the available states (3) into groups of 2 (one state printed on each pass). In this case, the number of fundamental density levels will be six: {0,0}, {0,S}, {S,S}, {0,L}, {S,L}, and {L,L}. The intermediate densities are again created by halftoning between the available density levels, but as someone skilled in the art will know, the more density levels there are to render an image, the better the image quality will be.
To produce some of the fundamental density levels, more than one nozzle must be activated for a given pixel location during the printing process. For example, in a two pass interlaced print mode, printing a state of {S,L} at a given pixel location on the page requires that both of the nozzles that pass over the pixel are activated. This violates the constraints of the above discussed methods for correcting for malperforming nozzles. Thus, a different method of correcting for malperforming nozzles is required to achieve improved image quality on modern inkjet printers.